1. Field of the Invention
The present invention relates generally to wireless communication system, and in particular, to a device and method for acquiring a phase of spreading code in a CDMA communication system.
2. Description of the Related Art
In a CDMA communication system, code synchronization between a transmitter and a receiver is very important. That is, the transmitter spreads intended data with a pseudo noise (PN) code and thereafter transmits the spread data. The receiver then calculates a correlation between the received PN code and a locally generated code generated by accurately synchronizing the received PN code with the locally generated code in order to restore the received data.
This synchronization procedure is generally performed in two steps: a code acquisition step and a code tracking step. In the code phase acquisition step, the received PN code is approximately synchronized with a locally generated code in the receiver to within a chip period of the PN code. In the code phase tracking step, the two codes are accurately synchronized with each other after the code acquisition.
In particular, for rapid synchronization, the code phase acquisition step is very important. The following references are listed for their relevance to code acquisition and are incorporated by reference herein:
Reference [1]: A. Polydoros and C. L. Weber, “A Unifiled Approach to Serial Serach Spread-spectrum Code Acquisition-Part I: General Theory,” IEEE Trans. Coommun., Vol. COM-32, No. 5, pp. 542-549, May 1984.
Reference [2]: W. Siess and C. L. Weber, “Acquisition of Direct Sequence Signals with Modulation and Jamming,” IEEE J. elect. Commun., Vol. SAC-4, No. 2, pp. 254-272, March 1986.
Reference [3]: A. Krzymien, A Jalali and P. Mermelstein, “Rapid Acquisition Algorithms for Synchronization of Bursty Transmission in CDMA Microcellular and Personal Wireless Systems,” IEEE J. Select. Areas Commun., Vol. 14, No. 3, pp. 570-579, Apr. 1996.
Reference [4]: Polydoros and C. L. Weber, “A Unified Approach to Serial Search Spread-spectrum Code Acquisition-Part II: A Matched Filter Receiver,” IEEE Trans. Commun., Vol. COM-32, No. 5, pp. 550-560, May 1984.
Reference [5]: B. Ibrahim and A. H. Aghvami, “Direct Sequence Spread Spectrum Matched Filter Acquisition in Frequency-selective Rayleigh Fading Channels,” IEEE J. Select, Areas Commun., Vol. 12, No. 5, pp. 885-890, June 1994.
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Reference [7]: E. Corazza, “On the MAX/TC Criterion for Code Acquisition and its Application to DS-SSMA Systems,” IEEE Trans. Commun., Vol. 44, No. 9, pp. 1173-1182, Sep. 1996.
Reference [8]: Li, and S. Tantaratana, “Optimal and Suboptimal Coherent Acquisition Schemes for PN Sequences with Data Modulation,” IEEE Trans. Commun., Vol. 43, No. 2/3/4, pp. 554-564, February/March/April 1995.
Reference [9]: Simon, J. K. Omuar, R. A. Scholtz and B. K. Levitt, Spread Spectrum Communications, Vols. I, II and III, Rockville, Md.: Computer Science Press, 1985.
Reference [10]: B. Ward and K. P. Yiu, “Acquisition of Pseudonoise Signals by Recursion-aided Sequential Estimation,” IEEE Trans. Commun., Vol. COM-25, pp. 784-794, August 1977.
Reference [11]: C. Kilgus, “Pseudonoise Code Acquisition Using Majority Logic Decoding,” IEEE Trans. Commun., Vol. COM-21, pp. 772-774, June 1973.
Reference [12]: B. Milstein, J. Gevragiv and P. K. Das, “Rapid Acquisition for Direct Sequence Spread-spectrum Communications Using Parallel SAW Convolvers,” IEEE Trans. Commun., Vol. COM-33, No. 7, pp. 593-600. July 1985.
Reference [13]: Sourour and S. C. Gupta, “Direct-sequence Spread-spectrum Parallel Acquisition in a Fading Mobile Channel,” IEEE Trans. Commun., Vol. 38, No. 7, pp. 992-998, July 1990.
Reference [14]: Sourour and S. C. Gupta, “Direct-sequence Spread-spectrum Parallel Acquisition in Nonselective and Frequency Selective Rician Fading Mobile Channels,” IEEE J. Select. Areas Commun., Vol. 10, No. 4, pp. 760-769, May 1992.
Reference [15]: Cheng, “Performance of a Class of Parallel Spread-spectrum Code Acquisition Schemes in the Presence of Data Modulation,” IEEE Trans. Commun., Vol. 36, No. 5, pp. 596-604, May 1998.
Reference [16]: Chawla and D. V. Sarwate, “Parallel Acquisition of PN Sequences in DS/SS Systems,” IEEE Trans. Commun., Vol. 42, No. 5, pp. 2155-2164, May 1994.
A serial search is a widely recognized code acquisition technique which calculates a correlation between the received code and the local code generated in the receiver and then determines the code phase acquisition according to whether the calculated value is higher than a threshold value or is equal to a maximum value. This technique can be classified as a form of Maximum-Likelihood (ML) search technique in series, as disclosed in the following Reference [9]. Although this technique exhibits superior performance in a multi-user environment as compared with a sequential estimation technique as disclosed in References [10] and [11], it exhibits poor performance as compared with parallel search techniques as disclosed in Reference [8] and [12]-[16]. However, use of the parallel search technique is limited because of its complexity.
In most cases, an actual code acquisition system uses both a searcher and a verifier in order to prevent a false alarm, as disclosed in References [4]-[7] and [12]-[15]. In this case, the searcher and the verifier alternately operate, sharing one correlator. That is, the verification is started, as the acquisition is determined (or declared) by the searcher, the searcher ceases to operate during the verification. The searcher operates again only when a false alarm is raised by the verifier. The procedure is repeated alternating between search and verification, until verification is successfully completed. Through such a procedure, it is possible to reduce a mean acquisition time. The conventional code acquisition system is particularly effective when the searcher has a high probability of false alarm. Such a system having the search and verification functions is referred to as a “two-dwell” system.
In the two-dwell system due to alternation between search and verification, search is not performed during a verification which was initiated in response to a provisional acquisition generated from a previous search operation. The search is resumed only when the false alarm is declared after verification. As such, there is a possibility that the actual code phase offset occurs during the verification period and it will not be detected since the search procedure is on hold during verification. This situation makes it difficult to acquire the code phase. In addition, the time required for code phase acquisition is increased as a result.